Applications for reading or amplifying signals of capacitive sources usually require very high impedance input terminal(s), since a lower impedance input may deteriorate the input signal. In such applications, a technique may also be needed to define a DC operating point for the application. This may require using an input component with some ohmic characteristic (i.e., the component can source or sink some current) as well as a very high impedance. For example, it may be desirable to design a MEMs-based microphone, for instance, to include input components with impedance values in excess of 1 GOhm, or even in excess of 100 GOhm. Additionally, it may be desirable for the input/impedance components to be as linear as possible, because non-linear impedance components may produce some rectifying effects, which deteriorate the desired operating point.
Some proposed solutions use a diode, for example, with an operating voltage of zero volts as a resistive component. This can be a simple and inexpensive solution, and at a zero-volt operating point a diode typically has a very high resistance. However, the resistance values of a diode may widely vary, and also may be quite nonlinear. Other solutions with improved linearity include complex closed-loop circuits involving, for example, analog-to-digital and digital-to-analog conversion and/or signal processing.